1. Field of the Invention
This invention relates generally to methods of dispersing carbon nanotubes in elastomers, the resulting elastomer compositions, and the articles made from the resulting elastomer compositions.
2. Description of the Prior Art
Because carbon nanotubes (“CNT”) are produced as entangled bundles of fibers, getting them to disperse in a polymer is a critical step in their utilization as reinforcing additives. A number of approaches have been used to obtain polymers with dispersed carbon nanotubes. Among these methods are melt blending of CNT into thermoplastic resins, polymerization in the presence of the CNT, high shear mixing, chemical modification of the CNT, and the use of surfactants.
Regarding mixing carbon nanotubes into rubber or elastomeric polymers, it is very difficult to disperse the CNT in a matrix material with as high a viscosity as most elastomers or rubbers have. The application of heat does not reduce the viscosity of the polymer significantly enough as it does in melt-blending into plastics. What is needed is a method to improve the dispersibility of the CNT in rubber.
U.S. Pat. Appl. Pub. No. 2012/0241686 A1 discloses an electrically conductive thermoplastic composition prepared by melt blending a thermoplastic polymer and a masterbatch of carbon nanotubes in wax having a melting point of about 45 to about 150° C. Use of the master batch of carbon nanotubes in wax also improves the melt flow properties of the electrically conductive thermoplastic composition. No mention is made of elastomer compositions.
U.S. Pat. No. 7,785,701 B2 discloses a carbon fiber composite material comprising an elastomer and a carbon nanofiber dispersed in the elastomer, wherein the elastomer has an unsaturated bond or a group, having affinity to the carbon nanofiber. When the affinity of the elastomer for the nanofiber is high, the dispersion is reportedly easy by the shear force of mixing, e.g. on an open roll mill. Dispersion is reportedly not so easy for nonpolar elastomers such as EPDM. The resulting mill-mixed compositions show an increase in modulus and strength but a decrease in elongation as is typical of many reinforcing fillers, relative to the composition without nanofiber.
EP 2,138,535 B1 discloses a vulcanizable composition containing a specific hydrogenated carboxylated nitrile rubber (HXNBR), a cross-linking agent and carbon nanotubes and a process for preparing such compositions. It is reported therein that solvent mixing, melt mixing and the spray drying process have been employed as processing methods to prepare some rubber/CNT composites. The examples of MWCNTs in HXNBR were conventionally mixed in an internal mixer and two-roll mill. The resulting compositions showed an increase in modulus and strength but essentially the same elongation.